The present invention relates generally to the field of oxygen storage and delivery systems, and, more particularly, to a system for safe, compact storage of liquid oxygen especially for safe, convenient transport in a vehicle such as a helicopter or an ambulance for ultimate delivery of gaseous oxygen to a patient.
Previously, land ambulances usually carried compressed gas cylinders, commonly referred to as "H" cylinders, a well-known type of steel tank, to store oxygen under high pressure for various uses, particularly in hospitals and manufacturing industry. Typically, oxygen is contained in such tanks at approximately 2,000 psig. These conventional tanks are available in different sizes, but the most commonly used variety weigh approximately 125 pounds and occupy a space at least approximately five feet high and about nine inches in diameter.
Due to their weight and elongated form conventional compressed gas oxygen cylinders are difficult and even dangerous to handle. These cylinders are so heavy as to affect the center of gravity of the ambulance. Furthermore, there exists the significant risk that a tank can be damaged in an accident, resulting in an explosion and turning pieces of the highly pressured cylinder into high speed projectiles.
In helicopter ambulances the weight and explosion concerns caused by compressed gas cylinders cannot be ignored. When liquid oxygen is used in aircraft the parameters of size, weight and explosion hazard acquire increased importance. As will be shown herein the new liquid oxygen system which has been developed with air ambulances in mind has beneficial features which make it equally useful in land ambulances. Accordingly, the new system will sometimes be referred to herein as the ALOXS (ambulance liquid oxygen system, or LOXS), for convenience.
Orbitally shaped oxygen tanks have been used for some time in military and commercial aircraft cryogenic systems for storage and delivery of oxygen to crew members. These strong round metal tanks generally have multiple walls and contain oxygen at approximately only 200 psig and thus are inherently safer than the compressed gas cylinders just described. They are also much lighter than compressed gas cylinders containing approximately the same volume of oxygen. For purposes of comparing weight and oxygen containing capacity of the new system with the above-mentioned H cylinders, as well as with other known oxygen cylinders, the following table is provided:
______________________________________ Approximate Weight And Capacity Comparison ALOXS Versus High Pressure Cylinders ALOXS Weight: 38.5 lbs. empty, 60.0 lbs. full ALOXS Capacity: 6580 liters of gaseous oxygen @ STP Weight Full Oxygen of Weight Capacity Equivalent Equivalent of of Number Number Cylinder Cylinder Cylinder of of Type Lbs. Liters Cylinders Cylinders ______________________________________ D 10.1 360 18.3 184.8 E 13.8 625 10.5 144.9 M 72.9 3,029 2.2 160.4 G 111.5 5,300 1.2 133.8 H 125.3 6,246 1.1 137.8 ______________________________________
Another hazard exists every time a cylinder is changed out. Should a high pressure cylinder be knocked over and the valve broken off a missile would be created which could injure persons nearby and damage equipment and facilities.
An additional concern in the area of safety relates to further potential injury to personnel. A fully charged H cylinder weighs well over 145 pounds. Most "EMS" (emergency medical service) personnel are already at high risk of back injury from lifting patients and do not need additional such stresses imposed on them. Ordinarily, the high pressure gas cylinder must be unloaded from the ambulance and a charged (full) cylinder loaded on, often without the aid of a hoist, winch, or dolly, every time the oxygen system needs to be resupplied.
The design of the ALOXS is such that it may be permanently installed on the emergency medical vehicle. For example, one extremely well protected position is beneath the module inside the chassis frame. An alternative position is within one of the equipment compartments of the module. This exposes the ALOXS to the potential for impact damage discussed above, but the ALOXS is inherently able to withstand such stress without creating a safety hazard.
Firstly, the new system is a low pressure system, 235 psig maximum, as opposed to the 2000 psig of a high pressure gaseous oxygen system; so the potential for explosion with the ALOXS is substantially non-existent.
Secondly, the ALOXS tank is fabricated of "304" stainless steel which is much more ductile and therefore better able to withstand shock and deformation than the alloy steel used in the manufacture of high pressure gas cylinders.
And finally, liquid oxygen is inherently safer than gaseous oxygen for most applications, and is definitely safer in this case. Should an ALOXS tank be penetrated, the contained liquid oxygen would merely spill to the ground, vaporize, and drift harmlessly away. By contrast, should a high pressure oxygen cylinder be penetrated, there would be a high velocity release of gaseous oxygen. It is common knowledge that many fires have been initiated and promulgated by high velocity gaseous oxygen flow.
When the ALOXS is mounted to the ambulance by either method described above there would be no lifting or hoisting of equipment to fill the system. The only lifting required would be to raise the fill hose to connect to the fill valve on the ambulance.
It should be noted that the ALOXS can be configured so that the tank can be easily and quickly removed from the ambulance for filling if, due to some unusual circumstance, that needed to be done. However, should this be the case, personnel would be working with only up to approximately 60 pounds with the new system, as opposed to approximately 145 pounds with a conventional high pressure gas system.
Thus, it has become apparent that there is a need for a safe, convenient system for storing and supplying oxygen particularly for use in emergency care vehicles such as helicopters and ambulances, which system is light weight relative to known oxygen storage and delivery systems and economical to manufacture and operate. The new oxygen system described below provides all these features and is well adapted for home health care and hospital use in addition to being ideally suited for aircraft life support. It has been found that orbital oxygen tanks can form part of a new liquid oxygen system to transport oxygen to patients by either land or air in a safe, facile and convenient manner.
The ALOXS described and shown in schematic form herein is a 6,580 gaseous liter capacity oxygen system which contains and stores oxygen in the form of 8.5 liters of liquid and supplies gaseous oxygen, on demand, at a nominal pressure of 50 psig and a minimum flow rate of 100 liters per minute at a temperature within 20 degrees Fahrenheit of ambient.
The nominal operating pressure of the ALOXS is 70 psig. As such, with the incorporation of the pressure regulator, the system supplies oxygen at 50 psig, the standard operating pressure of medical oxygen equipment.
The ALOXS contains a capacitance type quantity gauging system which provides users with a way to monitor the content of the storage tank. Tank contents are displayed by a quantity indicator having a light emitting diode display.
The ALOXS utilizes the saturated liquid principle of operation as opposed to the pressure buildup scheme. A saturated liquid system is more reliable since it utilizes fewer and more reliable components than those used in a pressure buildup system.
The new ALOXS ordinarily includes several specific features especially worth noting. For example, the quantity indicator includes a full level indicator circuit which provides servicing personnel an audible or visual signal when the tank full level has been attained. Also, during preliminary market survey work it became apparent that it would be beneficial to users if the system could accommodate a variety of filling pressures so that the system could be filled from a variety of sources such as a captive supply, a commercial industrial gas supplier, a home health care gas supplier, or from a hospital liquid oxygen system.
These unique features lend the ALOXS significant advantages in terms of operation, serviceability, durability, reliability, and safety when compared to other potentially competitive systems such as modified home health care units, industrial gas supply equipment, or aircraft life support systems.
The cost of oxygen varies from region to region depending upon proximity to a production plant, the local competitive situation and the like. It should be noted that because of the requirement that an ambulance have a minimum quantity of oxygen on board before responding to a call the usual H cylinder must sometimes by replaced when its pressure has been depleted to approximately 800 psig. Thus, approximately 20% of an H cylinder's volume is commonly paid for but not used. This expense can be obviated with the new system.
The new system is ideally compatible with filling pressures ranging from about 70 to about 235 psig and incorporates a filling scheme which accommodates these wide variations of pressure and allows the system to be filled from essentially any source. It incorporates a unique arrangement of valves and gauges so that the pressure difference across the system can be maintained at a constant level. A differential pressure gauge is critically added across the fill and vent circuits of the system and a needle valve is placed in the outlet of the vent circuit for controlling the pressure difference, to keep it at a constant level, as monitored by the differential pressure gauge, irrespective of the absolute filling pressure.
Thus, it is among the several advantages of the present invention that the new oxygen system has a fraction of the weight, significantly more "breathing" capacity, costs much less per cubic foot of oxygen and saves about three cubic feet of space, as compared to the conventional H cylinders.
It is further among the advantages of the present invention, having the features indicated, that it meets criterion for use in emergency medical service helicopters, while also being compatible with known home health care and hospital liquid oxygen equipment as well as being capable of being filled from a variety of sources.
Accordingly, in keeping with the above goals, the present invention is, briefly, a system for compactly safely storing and delivering oxygen which system has a plurality of elements interconnected by fluid lines and includes a reinforced, metal, orbitally-shaped tank which receives and contains oxygen to be stored as a liquid and delivered by the system to an end user, a filler valve in communication with the tank for providing oxygen from a main source thereof to the system, and a vent valve connected to the liquid oxygen tank for selectively releasing oxygen from the system. The new system also includes a differential pressure gauge located between and in communication with the fill valve and the vent valve to permit an operator of the system to thereby monitor the pressure differential in the system so that selective adjustments can be made in a timely and controlled manner to maintain the pressure within the system during filling at an optimum level. The system also has at least one pressure relief valve between and in communication with the oxygen tank and the vent, to thereby release pressure from the system as necessary to maintain the desired temperature and pressure conditions within the system, a heat exchanger in communication with and between the liquid oxygen tank and a pressure regulator and a fluid pressure regulator in communication with and between the heat exchanger and an oxygen flow control outlet. The system further includes a flow control outlet by which flow of oxygen from the system to an end user can be controlled, and a phase selector valve disposed in line between and in communication with the liquid oxygen tank and the heat exchanger, to thereby permit the system to select as a function of pressure whether oxygen supplied from the liquid oxygen tank to the heat exchanger will be supplied as either a liquid or a gas, the tank, filler valve, vent valve, differential pressure gauge, at least one pressure relief valve, supply heat exchanger, pressure regulator and phase selector valve all being sized and arranged in relation to one another so as to provide a light-weight, compact system for safely storing and delivering oxygen which is suitable for use by a home-bound patient as well as in a movable vehicle, and otherwise where safety, weight and size are of concern.
The invention further includes the above-mentioned features in combination with an emergency medical transport vehicle.
Further advantages of the invention will be in part apparent and in part pointed out hereinbelow.